Android退出应用程序的实现方法

import java.util.LinkedList; 
import java.util.List; 
import android.app.Activity; 
import android.app.Application; 
 
public class SysApplication extends Application{ 
 private List mList = new LinkedList(); 
 private static SysApplication instance; 
  
 private SysApplication() 
 { 
 } 
 public synchronized static SysApplication getInstance() 
 { 
 if(null == instance) 
 { 
  instance = new SysApplication(); 
 } 
 return instance; 
 } 
 //add Activity 
 public void addActivity(Activity activity) 
 { 
 mList.add(activity); 
 } 
  
 public void exit() 
 { 
 try{ 
  for (Activity activity : mList) 
  { 
  if(activity != null) 
   activity.finish(); 
  } 
 }catch (Exception e){ 
  e.printStackTrace(); 
 }finally{ 
  System.exit(0); 
 } 
 } 
 public void onLowMemory(){ 
 super.onLowMemory(); 
 System.gc(); 
 } 
}